optimizing buffer management for reliable multicast

Optimizing Buffer Management for Reliable Multicast

Zhen XiaoAT&T Labs – Research

Joint work with Ken Birman and Robbert van Renesse

Why important?

• Many applications desire reliable or semi-reliable delivery.

• IP multicast is best-effort.• Buffering is necessary for retransmission.• Buffer space is limited!

How to utilize the amount of buffer space most efficiently?

Previous Work

• RMTP: Buffer all messages on repair servers.– Impractical for long-lived sessions.

• SRM: Regenerate messages at the application.– Buffer management at the application level remains a

challenge.• Stability Detection: Buffer messages until they are

stable (i.e. received by all members in the group).– It takes a long time to achieve stability in a large multicast

group.• Bimodal Multicast: Buffer messages for a fixed

amount of time.– Optimization: buffer messages on a sub-group of members.

Talk Overview

• RRMP: Randomized Reliable Multicast Protocol• Error recovery algorithm in RRMP: Infocom 2001• Buffering algorithms in RRMP: DSN 2002

– Feedback based short-term buffering– Randomized long-term buffering

• Simulation results• Summary

RRMP: Randomized Reliable Multicast Protocol

Key idea: combine previous work on randomized error recovery with the Bimodal Multicast protocol and hierarchical error recovery similar to that employed by tree-based protocols.

• Group receivers into a hierarchy.• Do not use any repair server.• parent region: the least upstream

region of a receiver in the hierarchy.• Each receiver maintains group

membership information about receivers in its region and receivers in its parent region.

Two-phase Error Recovery

Assume a receiver p detects a message loss. • local loss: the loss affects a fraction of receivers in p’s region • regional loss: the loss affects all receivers in p’s region

Local recovery: a receiver tries to recover the loss from randomly selected neighbors.

Remote recovery: some receivers in the region request retransmissions from the parent region.

s

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routers

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Overview of Buffering Scheme

Local recovery

Remote recovery

Error Recovery

Long-term buffering

Short-term buffering

Buffering

Short-term buffering: when a message is first introduced into the system.

Long-term buffering: when almost all receivers in a region have received the message.

Not All Messages Are Created Equal!

Internet

idle message: no request for this message has been received for a time interval T. (T is the idle threshold.)Short-term buffering: buffer a received message until it becomes idle.Result: messages most needed in the system stay in the buffer longer.No extra traffic overhead!

n: the size of a regionp: the percentage of members in this region missing a messageThe probability that a member will not receive any request:

As n , this probability can be approximated by pe

idea: a member uses the retransmission requests it received as feedback to estimate how many members in the region still miss the message.

np

n)

111(

Feedback-based Short-term Buffering

Simulation Results

• Short-term buffering in a local region.– 100 members in the region, fully connected.– RTT between any two members: 10ms.– idle threshold: 40ms.

• Outcome of IP multicast: select a random subset of members to hold a message initially.– Measure how long these members buffer the message.

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#members holding a message initially

aver

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buffe

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time

(ms)

0 20 40 60 80 100 120 1400

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time (ms)

#mem

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#received#buffered

96 %

Why Long-term Buffering ???

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idleidle

idle

Sorry, you are out of luck!

p

q

Randomized Long-term Bufferingidea: provide long-term buffering for an idle message at a small subset of receivers in each region.Load balancing: spread the load of buffering across all receivers in a region.

Randomized algorithm: each member independently tosses a coin to decide whether to become a long-term bufferer.

C: the expected number of long-term bufferers.

Saving in buffer space: n / C

Network dynamics: message transfer

The probability that k members buffer an idle message for different values of C, the expected number of long-term bufferers.

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C

Pro

babi

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of n

o lo

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rm b

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%)

The probability that no member buffers an idle message decreases exponentially with C

How to find a long-term bufferer ???

s

p

q

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routers

receivers

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help!

Do you have the msg?

have the msg?have the msg?have the msg?

The search is over!

Search Overhead

• Evaluate penalty in recovery time due to search for a bufferer in a region with 100 members.– RTT between any two members: 10ms.– Assume a remote request arrives at a random

member.– Simulation repeated 100 times with different random

seeds.• Question I: how does the search time change

with the number of bufferers?• Question II: how does the search time changes

with the region size?

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#bufferers

sear

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Search time as the number of bufferers increases.

100 200 300 400 500 600 700 800 900100015

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region size

sear

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Search time as the size of the region increases

Summary

• Efficient buffer management is essential for reliable multicast in a large group.

• Two phase buffering to address variances in delivery latency in a large group.

• Retransmission requests can be used as feedback to allocate buffer space adaptively.

• Spread the load of buffering among all members in a group through randomization.